Removal of fouling species from brackish water reverse osmosis reject stream.

Brine disposal from reverse osmosis (RO) systems remains a major challenge for the desalination industry especially in inland areas where discharge options are very limited. Solutions will entail the introduction of economic treatment processes that will alleviate the brine's negative impact on the environment and reduce its discharge volume. Such processes could act as an intermediary treatment process for the recycling of the brine through an additional RO stage which, for brackish water (BW) desalination, could lead to saving valuable water while reducing the amount of brine discharge. In this context, the study at hand attempts to evaluate the effectiveness of a one-step chemical process for the treatment of BWRO brine. This study seeks to determine optimal operating conditions relative to type, ratio, and dosage of alkalizing chemicals, pH and temperature, for substantially reducing the concentrations of scaling parameters such as calcium, magnesium, silica, and strontium. The results indicate that precipitation softening at pH = 11.5 using combined chemical dosages of NaOH and Na2CO3 in a ratio of 2:1 leads to substantial removal of calcium and magnesium (>95%) and moderately high removal of strontium and silica (>71%).

Vapor-induced transfer of bacteria in the absence of mechanical disturbances.

Transfer of bacteria through water vapor generated at moderate temperatures (30-50°C) in passive solar stills, has scarcely been reported. The objective of this research was to investigate whether bacteria in highly humid atmospheres can get transferred through water vapor in the absence of other transfer media to find their way to the distillate. To achieve this objective, passive solar reactors were chosen as the medium for experimentation, and distillation experiments were conducted by spiking a pure bacterial culture (Escherichia coli, Klebsiella pneumonia or Enterococcus faecalis) in low mineralized water vs. highly mineralized water in the dark under moderate temperatures ranges (30-35°C, 40-45°C and 50-55°C). Results showed that bacteria indeed get transferred with the vapor in stills when not exposed to solar U.V. radiation. The trends observed were adequately explained by a zero-modified Hurdle-Poisson model. The numbers of cultivable bacterial colonies transferred were bacterial size, water type and temperature dependent with highest transfers occurring in E. faecalis>E. coli>K. pneumonia at the 40°C range in low mineralized water. Proper management strategies are recommended to achieve complete disinfection in solar stills.

Adsorption of arsenate on untreated dolomite powder.

Raw dolomite powder was evaluated for its efficiency in adsorbing As(V) from water. An experimental setup comprised of a fluidized dolomite powder bed was used to assess the impact of various test variables on the efficiency of removal of As(V). Test influents including distilled water (DW), synthetic groundwater (SGW) and filtered sewage effluent (FSE) were employed to assess the effect of influent parameters on the adsorption process and the quality of the effluent generated. Dolomite exhibited good As(V) removal levels for distilled water (>92%) and synthetic ground water (>84%) influents at all initial As(V) concentrations tested (0.055-0.600 ppm). Breakthrough of dolomite bed occurred after 45 bed volumes for DW and 20 bed volumes for SGW influents with complete breakthrough taking place at more than 300 bed volumes. As(V) removal from FSE influents was relatively unsuccessful as compared to the DW and SGW influents. Partial removal in the order of 32% from filtered sewage effluent at initial concentration of 0.6 mg/L started at 75 bed volumes and gradually stopped at 165 bed volumes. Varying degrees of As(V) adsorption capacities were observed by the different test influents employed, which indicate that the adsorption of As(V) is adversely affected by competing species, mainly sulfates and phosphates present in the influent. The adsorptive behavior of dolomite was described by fitting data generated from the study into the Langmuir and Freundlich isotherm models. Both models described well the adsorption of dolomite. The average isotherm adsorptive capacity was determined at 5.02 mug/g. Regeneration of the dolomite bed can be achieved with the use of caustic soda solution at a pH of 10.5.

The combined effect of step-feed and recycling on RBC performance.

The effect of step-feed and recycling on the efficiency of treatment in rotating biological contractors (RBCs) was investigated. Experiments were conducted on a RBC system consisting of two three-stage units. The two units were operated in parallel, one unit as a control with a single feed point and the other in a step-feed mode. Seven experimental runs were conducted, using a simple synthetic wastewater, in which hydraulic loading rates were maintained constant at about 0.094 m3/m2d and variable COD concentrations (800-1200 mg/L), NH3-N concentrations (104-116 mg/L), step-feed ratios (60/40 and 70/30), and recirculation modes. Results indicate that improvements in the treatment efficiencies of RBC systems may be attained by operating the system in a step feed mode as compared to a single point feed mode. Further improvements may also be obtained by inducing effluent recirculation to the inlet stage. In both cases, levels of improvement have shown to be more pronounced for NH3-N removals and DO residual concentrations as compared to COD removals. The combined effect of step feed and recirculation of system effluent on NH3-N removal efficiencies is appreciable. Comparing removal efficiencies between the control, step feed and the combined step-feed/recirculation modes shows an increase of about 26%. For COD removals, the effect is not that pronounced as the results for the combined modes of operation indicate a limited increase in overall removal efficiency of about 3%. Supplementing step feed by a recircutation system also markedly improved DO residuals in stages 1 and 2 and to a lower extent in stage 3 of the system. Changing the ratio in the step feed system from 60/40% to 70/30% exhibited a slight improvement in NH3-N removal efficiencies as well as improved residual DO concentrations but very limited effect on COD removals. The use of a simple synthetic wastewater in the experiments may limit extrapolation of the findings to actual operation.